Ruhan must attack the chosen player if able, not a planeswalker controlled by the chosen player.

9/22/2011

If, during your declare attackers step, Ruhan is tapped, is affected by a spell or ability that says it can’t attack, or hasn’t been under your control continuously since your turn began (and doesn’t have haste), then it doesn’t attack. If there’s a cost associated with having Ruhan attack the chosen player, you aren’t forced to pay that cost, so it doesn’t have to attack that player in that case either.

9/22/2011

If Ruhan isn’t forced to attack the chosen player for one of the above reasons but can still attack, you may choose to have Ruhan attack another player, attack a planeswalker, or not attack at all.

9/22/2011

If your turn has multiple combat phases, Ruhan’s ability triggers at the beginning of each of them. Ignore any choices made during previous combat phases.